Failure modes, adhesion

As can be seen, some larger fragments are seen in the sample held at elevated temperature, and sodium has segregated to the surface. Such segregation is very common in high temperature cured specimens, where sodium is often found at the failure surface in an adhesive failure mode. ISS/SIMS data from the adhesive side of a titanium-epoxy failure interface from a tensile test specimen are shown, in Figure 8. 

Fig. 4 Cohesive and adhesive failure modes in the adhesive bonded joint...

Another important consideration is the mode of failure experienced during the 180 peel test. Pressure-sensitive adhesives with internal strength relatively low in comparison to their adhesion to a test substrate will fail cohesively while the same type of adhesive with improved internal strength will often fail adhesively. This adhesive failure can be important for temporary applications but is of less importance for high strength, permanent uses. Some systems change from the cohesive to the adhesive failure mode as the amount of cure is increased, a point which will be illustrated later. 

CELLOSOLVEWacrylate, and benzophenone (40/60/5/2). When irradiated for 6.5 seconds in air, a peel value of 5.5 pli is observed with cohesive failure while at 7.0 seconds it has changed to adhesive failure (4.0 pli). The maximum peel value for these formulations occurs just before the adhesive failure mode begins (point C->A, Figure 6-C). ... 

The SEM results suggest cohesive failure in the lm2 517 and lmp2-516 series because of the absence of metal substrate structure. The lower lap-shear strength of the lmp2-516 series compared to the lm2-517 series is attributed to a difference in the cohesive strength of the two polyimide resins. An adhesive failure mode is suggested for the 2m2-515 series of fracture samples from the appearance of metal substrate. 

Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

Fig. 1, Schematic of commonly u.sed methods for testing the strength of adhesive joints, (a) Peel test. Note that the peel angle can be changed depending on the test requirements, (b) Double overlap shear test. In this test, the failure is predominantly mode II. (c) Single overlap shear test. In this test the failure mode is mixture of mode I and mode II. (d) Blister test.

XPS analysis (Fig. 6), in conjunction with SEM examination of the failed debonded sides, identified the true modes of failure. The SAA control (hydrated oxide on both sides under SEM high A1 and 0 levels on both sides) failed within the oxide. Examination of the specimen treated with multilayer-forming 5000 ppm NTMP solution (distinct "metal" and "adhesive" sides under SEM high A1 and 0, low C levels on "metal" side high C, low A1 and 0 levels on "adhesive" side) indicated that the failure occurred between the metal and the adhesive (i.e., adhesive failure). 

Fig. 4. Illustration of the failure modes of the eombination of ICIOOO -SubalV pads. The root cause is the slurry between the pads, which can form large particles or break down the adhesive.

Major causes for coating failure are surface cracking and undetected pinholes or voids. These can be repaired and serious problems avoided. Coatings generally fail in different modes, these are chemical failure, abrasion failure, adhesive failure, cohesive failure and undercoat corrosion. For performance evaluation of coatings on experimental basis on these parameters various ASTM and BS specifications are presently being used. 

Two types of composite physical property tests were conducted to measure properties which are sensitive to the degree of adhesion and failure mode of the fiber-matrix interphase. Short beam shear tests (ASTM D2344-84) were conducted on 18 ply unidirectional laminates. The support span-to-thickness ratio... 

The strength of adhesion between the fiber and matrix could also be expected to play a role in this change in failure mode. The interfacial testing system (ITS) provides comparative data on the interfacial shear strengths of the bare and sized E-glass fibers in real composites. A handbook value of 76 GPa [19] was used for the tensile modulus of E-glass fibers and the matrix shear modulus was previously determined as 1.10 GPa. Table 4 lists the mean interfacial shear strength, standard deviation (SD), and number of fiber ends tested for the two fiber types. 

The results reported here are similar to a study recently completed in which the adhesion of carbon fibers to epoxy matrices was varied [25, 26]. Over three different levels of adhesion and three different failure modes, composite properties both in the fiber direction and perpendicular to the fiber-matrix interface were shown to be dependent both on the level of adhesion and on the interphase properties and failure modes. 

The modulus and toughness of this interphase combined with the increased fiber-matrix adhesion can be used to explain the resulting mechanical properties of these composite materials. For both interphase-sensitive and -insensitive mechanical properties, it has been concluded that the strength of the interphase and the failure mode initiated by the interphase properties can be responsible for composite mechanical properties. 

There is an apparent optimum relative humidity level required to achieve good adhesion and durability. Priming the steel adherends at 18% RH caused failure in the wedge samples within the steel (oxide) layer. Adherends primed at 34% RH failed within the alkoxide primer layer, whereas at 51% RH failure occurred primarily within the adhesive layer. This change in locus of failure with humidity was not evident using the wedge crack test when the adherends were primed with aluminum alkoxides. A peel-type test would probably be more sensitive in detecting these shifts in failure mode. 

Adherends Adhesive Tensile shear strength, MPa Failure mode... 

A diagram that one might use to illustrate a possible set of experimental data to represent all failure modes of an adhesive joint is presented in Fig. 15.1. When the data are closely analyzed and the extent of ultimate service life and proper safety margins are specified, the critical failure mode and time can be defined by identifying the weakest link — in this case the corrosion mechanism. If this predicted life is longer than the expected service life of the product, then the material specified for the adhesive joint can be qualified for use. 

A standard test report usually documents the resulting measurements, such as tensile shear strength and peel strength. It should also indicate all the pertinent conditions that are required to ensure reproducibility in subsequent testing. It is often very useful to describe the failure mode of the tested specimens. An analysis of the type (or mode) of failure is an extremely valuable tool to determine the cause of adhesive failure. The failed joint should be visually examined to determine where and to what extent failure occurred. The percent of the failure that is in the adhesion mode and that in the cohesion mode should be provided. A description of the failure mode itself (location, percent coverage, uniformity, etc.) is often quite useful. The purpose of this exercise is to establish the weak link in the joint to better understand the mechanism of failure. 

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